Three-dimensional thin film solar cell substrates afford cost, performance, and mechanical strength advantages. Compared to traditional flat solar cells with a similar amount of silicon, 3-D TFSCs have superior mechanical strength, better light trapping, and lower cell processing costs because of their self-aligned nature.
BIPV involves the process of integrating electricity-producing photovoltaic technology into residential, commercial, and industrial building and construction designs and materials. With BIPV, the solar electricity-producing components actually become an integral part of the building or construction materials and design, often serving as the protective exterior weathering skin and/or interior building components. Semi-transparent or see-through solar PV modules comprise the most attractive segment of BIPV applications. These modules can be used in many applications including window glazing in building windows. In some applications they are also a part of shading devices such as car parking covers. Such BIPV systems are also known as “shadow-voltaic” systems. See-through BIPV modules can be also part of energy efficient glazing, where they are used instead of usual glass.
Currently, about 80% of the BIPV applications are served by crystalline silicon cell technology, while inorganic thin-film (TF) technologies account for the remaining 20% of the total BIPV market. However, the TF technologies are projected to capture over 50% of the BIPV applications by 2015. The TF technologies include amorphous silicon (a-Si), cadmium telluride (CdTe), copper-indium-gallium-diselenide (CIGS), and organic PV (OPV). Among them, CdTe and CIGS promise higher efficiencies than a-Si. However, these TF technologies in practice offer efficiencies in the range of 5% to 12%, with the TF see-through BIPV module efficiencies being essentially limited to the single-digit efficiency range of 4% to 8%. Both OPV and dye-sensitized solar cells (DSSC) are considered to be the third generation BIPV technologies (both currently providing module efficiencies on the order of 6%). All the TF and DSSC BIPV technologies currently offer much lower efficiencies than crystalline silicon BIPV. However, the TF technologies provide better aesthetics than crystalline silicon, particularly for see-through BIPV module applications. In a typical see-through crystalline silicon BIPV module for solar glass applications, the crystalline silicon cells are spaced apart to allow for visible light transmission in between the tiled cells. While these see-through crystalline silicon PV modules can provide relatively high effective efficiencies (e.g., typically in the range of 10% to 12%), they do not offer very attractive aesthetics, both due to the tiled design and also due to the standard busbar emitter interconnects in the cells (which may show visible metallization fingers and busbars).
DSSCs operate based on the interaction between light and a dye coated onto small grains of titanium dioxide. The grains are placed in a liquid that acts as an electrolyte, collecting the electrons released by the dye as it absorbs light, thus generating current. The whole mixture is sandwiched between a transparent glass sheet electrode doped with tin oxide to make it electrically conductive, and a rear panel. The efficiency of DSSCs designed for outdoor conditions is currently about 6%. This is far below the efficiency of standard crystalline silicon BIPV modules.
OPV and DSSC BIPV modules cannot easily compete with the conventional crystalline silicon or TF BIPV solar panels due to their relatively low conversion efficiencies and shorter operational lifetimes. Crystalline silicon solar cells and modules have proven long lifetimes in excess of 25-30 years in the field, and no TF or DSSC technology can offer or match such a track record. While the conventional crystalline silicon wafer BIPV is suitable only for rigid BIPV applications, the TF and DSSC BIPV modules can be used for both rigid and flexible substrate applications.